In some circumstances, it can be desirable to leach mineral values from subterranean formations without conducting any mining operations. Such a technique can be useful, for example, where the grade or location of the ore body or the geologic conditions make extraction of the mineral values by conventional mining uneconomical. In such a situation, it can be desirable to leach the mineral values directly from the in situ subterranean formation.
In an exemplary in situ leaching operation, two or more wells are drilled to the portion of the subterranean formation containing desired mineral values. A leachant or lixiviant can be introduced in one or more injection wells to permeate the subterranean formation and a pregnant solution containing dissolved mineral values can be withdrawn from one or more adjacent production wells.
For example, a plurality of wells can be drilled several hundred feet to an ore body containing copper minerals to be leached. These can be oxidized copper minerals which are considered "soluable" since they can be dissolved in sulfuric acid solution, or "insoluable" copper minerals such as sulfides which require oxidation before they can be dissolved. A sulfuric acid solution, which can also contain ferric sulfate or the like for oxidizing insoluble copper minerals, is introduced as a leachant through one or more such injection wells. Sufficient pressure is maintained in the injection well that the leachant permeates through the copper ore body to one or more production wells. A pregnant solution containing copper values leached from the subterranean formation is withdrawn from the production wells. The leach solution can flow through the formation due to inherent permeability in the formation or at least in part due to artificially induced permeability. Such artificial permeability can be induced in the subterranean formation by hydraulic fracturing, for example.
The inherent permeability of many subterranean formations can be rather low and to obtain a reasonable volume of flow of liquid through the formation, rather high pressure gradients must be employed between injection and production wells. An injection well can, for example, have a wellhead pressure of several hundred psi. This pressure is superimposed on the hydrostatic head of the column of liquid in the well. In an embodiment where flow from a production well is induced by pressure applied to an injection well, the production well has the hydrostatic head of pregnant solution in the well. Alternatively, the pressure in a production well can be substantially less than the hydrostatic head where a submersible pump or air lift is employed for withdrawing pregnant solution.
After a well for in situ leaching has been drilled, it may be completed with fiber reinforced plastic pipe as a well bore casing. Such glass fiber reinforced plastic pipe is inserted in the well bore and the annulus between the pipe and surrounding formation is closed by pumping a grout of Portland cement or the like around the casing. This provides a casing in the well bore which is resistant to leach solutions such as sulfuric acid.
It is desirable to localize the introduction or withdrawal of liquid between the well bore and surrounding formation. The casing is, therefore, perforated at desired elevation in the well. Standard perforating tools employing projectiles, explosive charges, cutters, or the like, as commonly employed in oil wells are used for perforating the casing in a well for in situ leaching. Hydraulic fractures are induced adjacent such perforations. Such fracturing is induced after isolating a section of the well bore at the elevation where fracturing is desired. This section can be isolated by a conventional packer when near the bottom of a well or by a conventional straddle packer when at an elevation remote from the bottom of the well. Hydraulic pressure is increased in the isolated section of the well until the fracture extends radially a desired distance. Such radial fractures can be formed at a plurality of elevations by perforating the well at such elevations and isolating each elevation with a conventional straddle packer or the like.
When a subterranean formation is hydraulically fractured, the orientation of the resultant fracture depends on the depth below the ground surface and geologic factors. When formation is hydraulically fractured at depths less than about 1000 feet, fractures extend generally horizontally from the well. When formation is fractured at depths greater than about 3000 feet, the fractures extend generally vertically. Between about 1000 and 3000 feet below the surface, fractures can extend horizontally or vertically depending on local geologic conditions such as tectonic pressure, types of rock, presence of bedding planes, special measures taken to initiate fractures with a selected orientation, and the like. At a given elevation, fracture orientation can often be predicted or measured.
There are basically two different types of arrangements of injection and production well fractures. In the first, a plurality of radial fractures are formed by hydraulic fracturing around the injection well. When the depth of the well is such that generally horizontal fractures occur in the ore body, the fractures are formed at a plurality of elevations in the ore body. When the depth of the ore body is such that generally vertical fractures are formed by hydraulic fracturing, a plurality of vertical fractures can be formed extending in radial directions from the injection well.
Similarly, a plurality of radial fractures are formed by hydraulic fracturing around the production wells. Preferably the fractures are artificially propped with corrosion resistant particles so as to remain open when fracturing pressure is removed and provide channels for liquid flow during leaching.
In the second type of arrangement, which is used at depths in which fractures extend generally horizontally, the casing in the injection well is perforated at a selected elevation in the formation to be leached. Sufficient hydraulic pressure is applied at such elevation for fracturing the formation surrounding the injection well, thereby forming a radially extending, generally horizontal fracture. The fracture is extended at least most of the distance between the injection and production wells. The fracture can extend beyond a production well so long as the casing in the production well is not ruptured at that elevation.
Similarly, the casing in the production wells is perforated at a different elevation than the elevation of perforation in the injection well. A hydraulic fracture is extended radially from each such production wells at the elevation of that perforation. The fractures generally extend most of the distance between the production and injection wells.
In the first arrangement, the fractures adjacent the injection and production wells are limited to prevent the formation of short circuit passages between fractures, i.e., a direct communication between the fractures associated with the injection and production wells. In the second arrangement, the injection and production fractures overlap in horizontal extent and leave a zone of unfractured formation at elevations between the fractured zones. Thus, in each case, a zone of unfractured formation separates the two fractured zones. The fractured zone associated with the injection well essentially forms a fluid manifold for introduction of leachant to the zone of unfractured formation between the injection and production wells. Similarly, the fractured zone associated with a production well serves as a fluid manifold for withdrawing pregnant solution from the zone of unfractured formation.
It is important that there are no short circuits, passages through the unfractured formation, providing direct fluid communication between injection and production fractures so that liquid is caused to flow through the zone of unfractured formation between such unconnected fractures. However, such direct fluid communication occasionally occurs due to natural fissures or channels through unfractured formation, or when injection and production fractures inadvertently intersect. Such could occur, for example, if leachant pressure is higher than the fracture extension pressure. Fluid communication between injection and production wells can also occur through a portion of the formation having an inherently high natural permeability or through a portion that develops high permeability as a consequence of leaching minerals from the formation.
Direct fluid communication between injection and production fractures can lead to the bypassing of leachant and dilution of pregnant solution with leachant. Portions of the formation may not be leached if direct communication occurs.
It is therefore desirable to reduce or eliminate direct liquid flow between injection and production fractures whether due to inadvertent intersection of such fractures, high permeability regions of the formation, or naturally occurring fissures in the formation. Such a technique for reducing liquid flow is preferably enhanced or at least not degraded by leach solutions, such as the sulfuric solutions commonly used for leaching copper minerals.